Radio Frequency Identification (RFID) systems are commonly used to locate and track items in a near-field communication network including a reader device and at least one wireless terminal, or tag. Energized time-varying electromagnetic radio frequency (RF) waves, which comprise the carrier signal, are transmitted from the reader to the tags in a given RFID network or system. Inductive coupling may be used to transfer energy from one circuit (such as a conductive antenna coil and associated circuitry) to another by means of mutual inductance between the two circuits. A voltage is induced in the tag that can be rectified and used to power the tag circuitry. RFID networks may include tags and readers which exchange information using such inductive coupling between their inductive coupling coils (or antenna coils). To enable data to be passed from the tag to the reader, the tag circuitry changes or varies the load, which is referred to herein as a coupled impedance associated with the inductive coupling coil or element. This change can be detected by the reader as a result of the mutual inductive coupling, whereby a reader-originated RF signal can be modified by the tag to transmit encoded data.
FIG. 1a depicts a prior art RFID system in which data transmission from tags 101a-c to reader device 103 is performed on a same frequency channel or spectrum 104. Using the established inductive coupling technology, each of the plurality of tags typically in the RFID system or network sends RF signals using the modified carrier signal. Hence, the modified RF signals from each tag overlap those of other tags within the same RF frequency spectrum associated with a given reader device in the RFID network.
As a consequence, tag collision in RFID systems occur when the multiple tags are energized by the same RFID reader device, and simultaneously modify their respective, overlapping signals back to the reader using the given frequency channel. Thus the tag collision problem is exacerbated whenever a large number of tags must be read together in the same RF field. The reader is unable to differentiate these signals when the simultaneously generated signals collide. The tag collisions confuse the reader, generate data transmission errors, and generally reduce data throughput within the RFID system or network.
Various systems have been proposed to isolate individual tags. For example, in one technique aimed at reducing collision errors, when the reader recognizes that tag collision has taken place, it sends a special “gap pulse” signal. Upon receiving this signal, each tag consults a random number counter to determine the interval to wait before sending its data. Since each tag gets a unique number interval, the tags send their data at different times. The adverse impact on overall RFID system performance, in terms of data throughput rate, however, still exists.
Modulating the signal received by the tag and inductively coupling the modulated signal to the reader device is known, using such signal modulation schemes as phase shift keying (PSK) and amplitude shift keying (ASK), where the tag changes its associated impedance by changing the impedance match between states. However, the adverse effects of tag collisions resulting from overlapping modified signals on a given frequency channel still remain when using these known signal modulation schemes.
Moreover, especially pertinent in the context of a reader device of an RFID network is the effect of the DC offset in the reader device and the effects of the reader's phase noise.
In an inductive coupled RFID system, the underlying coils are defined by their physical size and structure. It is well know that a coupling system of the two coils can replaced by an equivalent transformer. The connection between these two coils is given by the magnetic field (B) and the underlying value to describe this connection is the mutual inductance (M) and/or the coupling factor (k).
FIG. 1b shows a prior art inductive coupled RFID system. The applicable Biot-Savart relationship is:
      B    →    =                              μ          o                ⁢                  i          1                            4        ⁢                                  ⁢        π              ⁢                  ∮        S            ⁢                                                  ⅆ              s                        →                    ×                      x            →                                                                          x              →                                            3                    This allows the calculation of the magnetic field at every point as function of the current, i1, as well as the geometry. In this equation (1), u0 describes the permeability, x stands for the distance and S describes the integration-path along the coil.Besides this, the mutual inductance and the coupling factor are given by:
      M    =                  ∫                  A          2                                              ⁢                                    B            ⁡                          (                              i                1                            )                                            i            1                          ⁢                                  ⁢                  ⅆ                      A            2                                    k    =          M                                    L            1                    ⁢                      L            2                              Here A2 describes the area of the second coil, while L1 and L2 describe the inductance of the two coils. The distance between the reader-coil and transponder-coil also determines the coupling factor.
Still with reference to FIG. 1b, the impedance as seen by the reader device is:Zin=ω2M2[Y1+Y2]where ω is the operating frequency in rads/s, M is the mutual inductance, Y1+Y2 are the admittances within the tag device. Here, L1 and L2 are in resonance with the capacitors C1 and C2, respectively. The Y1+Y2 admittances are modulated to transfer information back to the reader. The Y1+Y2 are either modulated via amplitude (ASK) or in-phase (PSK). Y1+Y2 can also be modulated using multi-phase PSK and multi-amplitude ASK, but this poses an issue on the Q of the resonance of L1C1 and L2C2.
The admittances Y1+Y2 are modulated such that most of the data in frequency domain sits near DC. This poses a problem for the reader device since it has to distinguish the actual signal from DC offsets that may be produced by the reader itself; for example, the operating frequency of the reader leaks back into itself producing a DC, or the phase noise of the oscillator used in the reader becomes (undesirably) superimposed on the modulated signal.